Elements of the platinum group such as platinum and rhodium, and cerium oxide having an oxygen storage effect for enhancement of their activity at low temperature are mainly used now as catalytic converters to simultaneously remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in exhaust gas exhausted from internal combustion engines such as automobiles.
However, it is said that in catalysts containing an element of the platinum group and cerium oxide the oxygen storage function of cerium oxide strikingly lowers at a high temperature exposure above 800.degree. C. and thus the catalysts are liable to deteriorate at those temperatures. Thus, many methods have been disclosed comprising adding oxides of alkaline earth elements, or zirconium compounds for the purpose of inhibiting crystallization of cerium oxide and preeserving its oxygen storage effect (for example, Japanese Laid-Open Patent Publication Nos. 58347/1989 and 116741/1988).
Further, in the latest catalytic converter, catalysts having a two or more-layer structure and containing rhodium in the outer layer are becoming conventional and permit effective utilization of expensive rhodium. It is known in these catalysts that it is effective to add zirconium oxide for prevention of lowering of the catalytic performance due to interaction of rhodium with alumina. Further, in these catalysts, it is considered that the catalytic performance is enhanced by an interaction between rhodium and zirconium oxide (for example, Japanese Laid-Open Patent Publication Nos. 222,539/1986 and 88,040/1988).
However, when zirconium oxide is exposed to high temperatures, its crystalline structure changes from the metastable tetragonal system having a catalytic activity to the inactive monoclinic system, contribution of rhodium and zirconium oxide to catalytic performance due to their an interaction diminishes, and the catalytic performance is lowered. At present there is no known catalyst to stabilize zirconium oxide in the catalyst and preserve the an interaction between rhodium and zirconium oxide.